1. Field
The invention relates to a method and an apparatus for preserving compatibility between legacy mode of operation and new mode of operation in a communication system, wherein allocation of resources is different in the legacy mode of operation and the new mode of operation.
2. Background
Communication systems have been developed to allow transmission of information signals from an origination station to a physically distinct destination station. In transmitting information signal from the origination station, the information signal is first converted into a form suitable for efficient transmission over a communication channel. Conversion of the information signal involves varying a parameter of a carrier wave (carrier frequency) in accordance with the information signal in such a way that the spectrum of the resulting modulated carrier wave is confined within a pre-determined bandwidth. At the destination station, the original information signal is reconstructed from the carrier wave received over the communication channel. In general, such a reconstruction is achieved by using an inverse of the modulation process employed by the origination station.
Although from a theoretical standpoint entire electromagnetic spectrum is available, only a portion of the electromagnetic spectrum is usable due to existing technical, commercial, and regulatory confines. Consequently, the range of useful and available frequencies is finite, and; therefore, limited. As such, the usable portion of the electromagnetic spectrum represents a limited resource.
Originally, communication systems allowing users to communicate with one another comprised point-to-point systems, in which two users communicated with each other. In order that each user of the point-to-point system could talk to the other user at the same time, each user would be allotted its own communication channel for transmission, which will be used by the other user for reception. The communication channel comprises a portion of the electromagnetic spectrum.
According to one approach, the available electromagnetic spectrum resource is divided into separate, non-overlapping frequency bands each centered over an associated carrier frequency. Each of such separate, non-overlapping frequency bands comprises a communication channel. As illustrated in FIG. 1, a first communication channel comprising frequency band 110 is allocated to one user (not shown). The frequency band is centered about a first carrier frequency 112. A second communication channel, allocated to another user (not shown), comprises a frequency band 114, centered about a second carrier frequency 116. The width of each allocated frequency band 110, 114 (also known as the bandwidth) is dependent on the amount of information to be transmitted in a given period of time; the greater the information rate, the wider the bandwidth. Such an arrangement is known as frequency division multiplexing (FDM).
According to another approach, the available electromagnetic spectrum resource is divided into separate, non-overlapping frequency bands each centered over an associated carrier frequency, and each frequency band is further divided in a plurality of time intervals. A communication channel comprises a time interval or a plurality of time intervals in a frequency band. As illustrated in FIG. 2 (for one frequency band 200) a user (not shown) has use of the resource for communicating information signal for a first time interval 210(1) and another user (not shown) has use of that same frequency band 200 for communicating information signal for a second time interval 210(2). In the case of the above described point-to-point system—use of the resource would then be handed back to the first user for a third time interval 210(3), and so on with the use swapping between the two users as time progresses. It is noted that the time intervals 210(i), i=1 through N, may, but do not need to be of equal length. Such an arrangement is known as time division multiplexing (TDM).
It follows from the foregoing that, in a time division arrangement, the communication channel is discontinuous in time, e.g., a user may transmit only in the time intervals, in which the communication channel is allocated to the user.
In yet another approach to dividing the electromagnetic spectrum resource, the available electromagnetic spectrum resource is divided into separate, non-overlapping frequency bands, each centered over an associated carrier frequency, and in each of the frequency bands different communication channels are distinguished from each other by means of the use of codes. For each user, the code is combined with information signal before modulation on the electromagnetic carrier frequency. The codes are applied in such a manner that the information signal is spread over the whole of the available frequency band of the electromagnetic spectrum resource. Thus, instead of being distinguished by frequency or time, individual communication channels are defined by codes. As illustrated in FIG. 3 (for one frequency band 300), each of the two users (not shown) in a code division multiplex (CDM), is assigned one of codes 310(1), 310(2), 310(n), each of the codes being uniquely distinguishable from all other codes.
As the demand for communication has grown, the limited electromagnetic spectrum could not accommodate the number of communications and communication systems using a point-to-multipoint topology for bidirectional communication have been developed. Such communication systems, in which a plurality of users—multipoint—are communicating concurrently and bidirectionally with or through a common communication point, are exemplified by dispatch systems and mobile telephone communication systems (both terrestrial and satellite based).
To manage a multiple-access in such communication systems, i.e., the capability of a common communication point (a base station), to function as a portion of a communications link between more than a pair of the multiple users on subscriber stations concurrently, each of the above-discussed techniques for electromagnetic spectrum allocation was utilized, resulting in communication systems utilizing Frequency-Division Multiple Access (FDMA), Time-Division Multiple Access (TDMA), and Code-Division Multiple access (CDMA).
The above-referred multiple-access communication systems are complex and different parts of a communication system are supplied by different manufacturers. Consequently, different manufacturers' products must be compatible with one another. Compatibility is achieved by way of agreement between manufacturers, resulting in standards being defined.
With the viability of all the above-referred multiple-access techniques, there are numerous standards that have been adopted in different parts of the world. In Europe, one of the standards adopted for second generation (2G) cellular communication systems is the so-called GSM standard, which is based on a combination of the above discussed FDMA and TDMA techniques. In contrast, in the United States and Korea a CDMA standard known as IS-95 (“TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wide-Band Spread Spectrum Cellular System”) was adopted for the 2G cellular communication systems. Other countries, including the United States and Europe, have several different cellular communication systems and have adopted various different standards depending on the choice of the cellular communication system owners.
The development of a standard is a relatively slow process. This is becoming more problematic as the rate of technological advances increases. No sooner are standards set than they become out-dated and new standards have to be defined to take their place. An important consideration in developing a new standard is backward compatibility with existing—legacy—communication systems as well as envisioned evolution, if only because owners and users of the existing communication systems do not expect their equipment to become redundant and unusable in a short period of time.
The requirement for backward compatibility is also hindering evolution of existing standards in what should be a fast-developing environment. When a new standard is being developed, it is necessary at a very early stage of the standardization and regulation process to establish many attributes of a communication system, e.g., frequency band, (multiple) access method, communication channel(s) waveform, and the like. At this time, potential advances of technology during the development of the communication system as well as during the lifetime of the standard may usually not been foreseen.
The evolution of a third generation (3G) standards has already shown that enhancements of certain attributes of existing mode(s) of operation or an introduction of new attributes is of such a extent, that this enhanced—new mode of operation—must sometimes be delayed and/or complicates maintenance of compatibility with the existing—legacy—mode(s) of operation.
Additionally, new resource-sharing techniques are beginning to be developed and adopted. As an example, an OFDM (orthogonal frequency division multiplexing) is a technique, in which digital information is distributed over a large number of carriers that are spaced apart at precise frequencies.
Additionally, beam-forming techniques and multiple-input-multiple-output (MIMO) techniques are used to implement a space division multiplexing (SDM). These new techniques may be used alongside of or in a combination with such other techniques as FDMA, TDMA, and CDMA in order to optimize use of the available resource.
In a way of an example of the above-mentioned evolution, the 3G evolution of the IS-95 standard is known as CDMA2000 (“TR-45.5 Physical Layer Standard for CDMA2000 Spread Spectrum Systems”). Communication systems built to this CDMA2000 standard are backward compatible to the extent that IS-95 user equipment may be used therein. In Europe the GSM networks are being replaced by equipment built to a standard known as W-CDMA, which is a CDMA-based standard (“3rd Generation Partnership Project” or “3GPP”, see for example document nos. 3GPP TS 25.211, 3GPP TS 25.212, 3G TS 25.213, and 3GPP TS 25.214). CDMA2000 equipment and 3GPP equipment are not compatible with each other for a wide variety of reasons, including the bandwidth (1.25 MHz for CDMA2000 vs. 5 MHz for W-CDMA), frequencies and codes used by each station. As mentioned, such an incompatibility is undesirable.